Jeff Adams of Washington Sea Grant called attention to this video with the following comments:

Whenever you look at a crab molt the “how does it do that?” factor is pretty high. Here’s a video of a crab molting that’s pretty amazing. Some of you may recall Greg Jensen saying the crabs consume much of their own muscle and the old shell in preparation for molting which actually makes it easier to get out of the old one. … Still… “how do they do that!?”

I wasn’t able to verify the origin of this video, but the information I found on the internet is:

Enoshima Aquarium (Fujisawa, Japan) has released some time-lapse footage of a molting Japanese spider crab (Macrocheira kaempferi), whose 3.8 meter (12 ft 6 in) leg span makes it the world’s largest arthropod species. The video was shot over a 6-hour period.